This invention relates to a method for overcoming the problem of the mudding-ofi of productive formations in wells, particularly oil wells and gas wells, and for restoring by chemical treatment the porosity of the wall surfaces of oil, gas, or other wells, which wall surfaces have been sealed or mudded off during the operation of drilling the wells by the rotary method.

The rotary method ,of drilling wells is well- .known. It is described, for example, in U. S.

' by the drill bit as drilling proceeds, and may be dislodged therefrom only with difficulty. In the case of low-pressure formations, satisfactory removal of the mud solids may be and commonly is substantially impossible to achieve by any of the methods now practised.

It has been proposed that the problem may be overcome by subjecting the mud sheaths to the action of a reagent, e. g., a mineral acid, in such cases where the drilling mud contains sumcient amounts of a material which is capable of reacting with the first-named material, e. g., carbonates. It has also been proposed to. incorporate in the drilling fluid a material which may be subsequently reacted upon by a reagent, with the result that such chemical reaction will disintegrate the mud sheath or deposit and dislodge it from the formation walls. This latter method has been described in one practical form in U. S. Bureau of Mines Report of Investigations No. 3249, June, 1934. The authors of that report propose to incorporate in the drilling fluid a proportion of limestone or calcium carbonate, and subsequently dissolve such material by reacting it with hydrochloric acid. While other mineral acids may be employed in this process broadly, it is customary to use hydrochloric acid, usually in 10-15% concentration.

I have found that, while such proposals thee retically supply simple and satisfactory means for preventing the undesirable residue of mud solids from persisting on the walls of the wall bore and interfering with the productivity of formations penetrated by such bore, in practice the results are not as good as is desired. Ibepresent on the carbonate particles.

lieve the reason for the shortcomings of the proposals lies in the fact that the acidic solution employed to react with the limestone or calcium I carbonate, initially present in or incorporated in the drilling mud, is unable to contact such carbonate efiectively, due to the coating of oil I have discovered that if the acidic reagent employed to react with the carbonate particles is of the nature disclosed hereinafter, the effectiveness of the acid is greatly enhanced.

In its broadest aspect, my process does not require that the mud sheaths to be subjected to the action of my reagent shall contain carbonates or similar reactive materials, either intentionally added to, or naturally present in, the drilling mud used. Neither does it require that the reagent employed shall act chemically on the mud sheath, or on some component thereof, in order to be effective. On the contrary, the reagent may be successfully used in instances where the mud sheaths are entirely devoid of, or contain only insignificant proportions of carbonates orsimilarly susceptible materials; and the reagent may operate by virtue of certainobscure physical or physico-chemical relationships set up or existing between the reagent and the mud sheath, rather than by any strictly chemical reaction.

The reagent which I propose to use for this purpose consists of a relatively stable aqueous dispersion, in which the disperse phase consists of a water-insoluble organic liquid capable of acting as an oil solvent, and the continuous aqueous acidic phase contains a mineral acid, the dispersion being stabilized by an organic disparsing agent capable of producing an oil-inwater dispersion. Such reagent,'I have found, is capable of removing the oil coatings from the potentially reactive particles (e. g., carbonates) present in the mud sheaths, and leaves such particles more readily susceptible to the action of the reagent (e g., a mineral acid) employed in the aqueous phase of the reagent to disintegrate them.

In general, the more concentrated acid solutions cause a reduction in the dispersing power of most dispersing agents, as do electrolytes in general, for which reason it may be necessary to use more of the dispersing agent thamwould be required to produce a water dispersion of the same stability. It may be preferable to use more dilute acid in preparing the reagent, rather than, e. g., the 10-45% concentration of acid commonly employed, in order to obtain a relatively stable dispersion of the kind described and required. The dispersing agent is preferably characterized by the fact that its calcium and magnesium salts are water-soluble; but so long as undesirable precipitates are not produced from any reaction between the reagent and other materials present in the well (e. g., soluble calcium and magnesium salts produced from carbonates by, and dissolved in, the acid used), this is immaterial. The dispersingagent may be of such nature that it is capable of inhibiting the action of mineral acid on the well equipment and fittings; but this is utterly immaterial to the success of my process.

By "relatively stable aqueous dispersion", I mean one that is not spontaneously resolved into its components, on standing, for protracted Periods of time, c. 'g., for an hour or more.

I have found that satisfactory water-insoluble oil solvents are solvent naphtha, benzol, toluol, carbon tetrachloride, carbon bisulfide, tetralln, gasoline, kerosene, etc. The mineral acid I prefer to use is hydrochloric acid of concentrations up to 15%, although the concentration of the acid is not of vital importance, except that it should in general be kept below that concentration where the products resulting from the reaction with the mud solids become insoluble in the reagent. The organic dispersing agents that I have found most satisfactory are such materials as saponin, licorice,-casein, glue, peptone, etc. In general, the organic dispersing agent is characterized by the fact that it is capable of producing a relatively stable aqueous dispersion of the water-insoluble oilsolvents in the aqueous acidic material. It is not decomposed by mineral acids; or, if it is, such decomposition proceeds only very slowly. The ordinary well known organic dispersing agents are not all necessarily useful in preparing my reagent. For example,

When it is desired to disperse an oil solvent in ordinary water, this may be readily accomplished by means of such materials as ordinary soap,

, petroleum, sulfonates, sulfonated vegetable and animal oils, etc. However, when the aqueous phase, as in the present case, contains an acid,

' some of such dispersing agents (e. g., soaps) become useless, because they are decomposed by the acid. Others, I have found, become inoperative, because they are salted out of the aqueous phase by the acid. In such cases, I have found that they either fail to produce a dispersion at all, or else, actually produce a water-in-oil type of dispersion. While some of such conventional dispersing agents may be useless under all practical procedures, I have found that it is sometimes possible to use them by altering the acid concentration, the proportion of dispersing agent, or the phase volumes of the components of the dispersion.

As an example, only, of the reagent -I prefer to use in my process, the following may be cited: Three parts by weight of saponin, and 20 parts by weight of solvent naphtha are dispersed in parts by weight of 10% hydrochloric acid, using agitation, if necessary, to effect dispersion. A mixture of oil solvents may be employed, or a mixture of dispersing agents may be used, or a different mineral acid or mixture of acids may be used. The example is not intended to be exclusive. The proportions of ingredients, likewise, may be varied" within wide limits to suit individal cases to best advantage. While the reagent so prepared may cream, or show a concentration of dispersed phase at the top or th bottom of the container, the dispersed phase doe not separate as such within an hour or more.

In broadest scope, my process consists in apply ing a reagent of the kind described to a mu sheath in a well bore. The mud sheath may con tain sufilclent carbonate materials to permit th reagent to act chemically to decompose sucI carbonates, and so disintegrate the sheath. Th reagent maybe effective by reacting chemicall with other non-carbonate ingredients or compo nents of the sheath. It may react, in some man ner other than a strictly chemical one to accom plish the objective, 1. e., dislodge the mud sheath: A specific application of the process is found 11 instances where my reagent is appliedto sheath in which carbonate materials have been inten tionally incorporated in the drilling mud solid prior to their use, as described in the U. S. Burea1 of Mines Report of Investigations No. 3249, date June, 1934, previously mentioned. The followin example illustrates this specific procedure f0: applying my process, although in practising In: process the procedure may be varied to suit individual requirements.

The example ,will illustratethe general procedure without limiting the scope of the invention. An amount of limestone, crushed to pas: l50-mesh screen, is added tothe drilling fluid tc be used, in the proportion of 20-30 pounds pe: barrel. The well bore is then drilled by the rotary method, using the mud fluid so prepared After drilling, the mud fluid remaining in th hole is .washed out by circulating water until the returns are clear, or by bailing. Sufiicient of my reagent at least to cover the productive formation is then introduced into the well bore by merely pouring it into the casing, by means oi tubing, by means of a dump bailer, or by other means. Pressure may be applied to the reagent in the well by any desired means, such as an air compressor, or by injecting a head of oil into the well. In general, I prefer to keep the pressure fairly; low, to accelerate the formation and escape of carbon dioxide bubbles. The reagent is allowed to remain in the well until disintegration of the mud sheath is believed complete, and usually less than 24 hours; after which it is pumped? or preferably bailed out, along with the disintegrated mud sheaths. I have found that agitating the reagent in the well, for example, by means 'of a swab. greatly accelerates its action on the mud sheaths.

' Having thus described my invention, what I claim as new and 'desire to secure by Letters Patent is:

1. A process for removing mud sheaths from well bores, which consists in subjecting such sheaths to the action of a relatively stable aqueous emulsion of the oil-in-water type, in which the disperse phase is a water-insoluble oil solvent, and the continuous aqueous phase contains a reagent, capable of reacting chemically with a component of said mud sheaths, which dispersion is stabilized by an organic dispersing agent.

2. A process for removing mud sheaths from well bores, which consists in subjecting such sheaths to the action of a relatively stable oilin-acid emulsion, in which the disperse phase is a water-insoluble oil solvent, and the continuous aqueous phase contains a mineral acid, which dispension is stabilized by an organic dispersing agent. 4

3. A process for removing mud sheaths from well-bores, which consists in subjecting such sheaths to the action of a relatively stable oilin-acid emulsion, in which the disperse phase is a water-insoluble oil solvent, and the continuous aqueous phase contains hydrochloric acid,

which dispersion is stabilized by an organic dispersing agent.

4. A process for removing mud sheaths, from geological formations penetrated in the drilling of wells by methods involving the use of drilling fluid, which consists in incorporating in such drilling fluid a substance capable of reacting chemically with a given reagent; and subsequent to drilling the well with such fluid, subjecting the mud sheaths presentrin the well bore to the action of a relatively stable aqueous'emulsion' of- Dersing agent.

fluid, which consists in incorporating in such drilling fluid a substance capable of reacting chemically with a mineral acid; and subsequent to drilling the well with such fluid, subjectingthe mud sheaths present in the well bore to the action of a relatively stable oil-in-acid emulsion, in which the disperse phase is a water-insoluble oil solvent, and the continuous aqueous phase contains a mineral acid, which dispersion is stabilized by an organic dispersing agent.

' 6. A process for removing mud sheaths from geological formations penetrated during the drilling of wells by methods involving the use of drilling fluid, which consists in incorporating v .in suchdrilling fluid a substance capable of reacting chemically with hydrochlaric acid; and subsequent to drilling the well with such fluid, subjecting the mud sheaths present in the well bore to the action of a relatively stable oilin-acid emulsion, in which the disperse phase is a water-insoluble oil solvent, and the continuous aqueous phase contains hydrochloric acid, which dispersion is stabilized by an organic dis- LOUIS T. MONSON.